Boosting the sustainable recycling of spent lithium-ion batteries through mechanochemistry

Abstract

The rapid proliferation of spent lithium-ion batteries (LIBs) presents critical challenges to both resource sustainability and environmental sustainability. Conventional recycling methods are often limited by high chemical consumption, complex operations, and poor selectivity. Herein, we report a green and mechanochemically driven strategy for selective recovery of critical metals from mixed LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) and LiFePO₄ (LFP) cathodes. By coupling mechanical activation with the intrinsic redox properties of Fe(II)/Fe(III), the process induces controlled lattice distortion, phase transformation, and spontaneous redox reactions without external reducing agents. Density functional theory (DFT) calculations reveal that mechanochemical activation facilitates the formation of transition metal oxides (MeO, Me = Ni, Co, Mn) and FePO₄, enabling efficient liberation of target metals. Under optimized conditions, 99% Li recovery, >85% Ni, Co, and Mn recovery, and ∼90% Fe and P recovery are achieved using 0.16 M H₂SO₄. This low-energy, low-reagent process simplifies separation, minimizes secondary waste generation, and offers a scalable and sustainable pathway for closed-loop recycling of complex LIB waste, fully aligning with the principles of green chemistry and circular economy.

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Green foundation

1. This study proposes a green, mechanochemically driven process for selectively recovering mixed cathode materials from spent lithium-ion batteries, minimizing chemical input and eliminating high-temperature or reductant-intensive steps.

2. The process achieves 99% lithium recovery, over 85% selective separation of Ni, Co, and Mn, and 90% recovery of Fe and P, using only 0.16 M sulfuric acid under ambient conditions.

3. This method aligns with the principles of green chemistry by enhancing atom economy, reducing hazardous reagents, and simplifying operation. Future work will explore its scalability and adaptability to complex battery waste streams.

1. Introduction

Lithium-ion batteries (LIBs) are extensively used in portable electronic devices, energy storage systems, and electric vehicles (EVs) owing to their high specific energy, long cycle life, and negligible memory effect.¹ In 2025, global EVs sales have surpassed 20 million, accounting for over 10% of total vehicle sales worldwide.^2,3 The global EVs stock is projected to reach 200 million by 2030, representing a fourfold increase from current levels.^4,5 However, the scarcity of critical materials, particularly Li and Co, poses a great challenge to the sustainable growth of the current LIB industry.^6,7 Moreover, the improper disposal of spent LIBs poses serious environmental and human health risks due to the potential release of toxic heavy metals and organic electrolytes.⁸ Therefore, efficient recovery of the critical metals is essential for securing a sustainable supply of raw materials and mitigating the impact of hazardous materials.^9,10

Currently, recycling technologies for different cathode materials, like LiFePO₄ (LFP) and LiNi_xCo_yMn_1−x−yO₂ (N_xC_yM_1−x−y), can be broadly categorized into pyrometallurgical and hydrometallurgical processes.^11,12 Pyrometallurgy typically involves high-temperature treatment in tube furnaces with high throughput, simple operation, and broad applicability, converting the black mass into stable metal oxides or zero-valent metals.¹³ However, it generally allows the recovery of only high-value metals such as Ni, Co, and Cu, while Li remains in the slag and is difficult to recover. During this process, organic components such as the separator, binder, and electrolyte undergo combustion or evaporation, leading to the release of large amounts of harmful fumes.^14–16 Moreover, the resulting alloy products require further complex separation and purification, limiting the applicability of this method. In contrast, the hydrometallurgical process can effectively recover all metals by utilizing inorganic acids,^17,18 alkalis,^19,20 and organic chelating reagents^21,22 for the selective extraction of valuable metals.²³ Hydrometallurgy features lower energy consumption and enables high-purity metal recovery; however, when acid concentrations exceed process requirements or when impurities accumulate beyond acceptable limits, portions of the solution may need to be discharged as highly acidic wastewater, requiring further treatment.^2,24,25 Furthermore, the intensive use of concentrated chemical reagents and high energy input in conventional processes hinders the scalability of selective metal extraction from mixed cathode materials.²⁶ Therefore, technologies that minimize the use of chemical reagents, simplify the process, and reduce emissions are essential to meet the growing demand for economic, environmentally friendly, streamlined, and efficient recycling of increasingly complex LIBs.

The mechanochemical method has attracted increasing attention in the recycling of metals from spent LIBs due to its superiorities in terms of high efficiency and low chemical input, which can be achieved by disrupting the crystal structure to accelerate the separation.^27,28 Different co-grinding reagents, including salts such as Na₃Cit,²⁷ NH₄Cl²⁹ and FeCl₃,³⁰ as well as oxidants like NaClO³¹ and Na₂S₂O₈,³² have been used to enhance the disruption of the material structure, thereby facilitating subsequent metal separation. In addition, other types of co-grinding reagents such as Zn powder³³ and dry ice³⁴ have also been investigated. Salt-based reagents, under the induction of mechanical force, facilitate the substitution of Li⁺ in cathode materials with metal ions, thereby disrupting the cathode structure and enabling the selective extraction of Li⁺. In contrast, oxidizing agents drive chemical reactions under mechanical force to release Li⁺ during the oxidation of cathode materials, particularly LFP.³¹ These mechanochemical processes rely on the addition of chemical reagents, which complicates the subsequent separation steps. Therefore, we propose utilizing the redox activity of spent cathode materials themselves to drive structural transformation and metal separation without the need for external reagents. Our previous study developed a reagent-free mechanochemical approach that selectively recovers metals from LiCoO₂ (LCO) and LFP by leveraging their intrinsic redox properties (Co(III) → Co(II) and Fe(II) → Fe(III)). This method provides a low-reagent, simplified solution for the recycling of different types of spent cathode materials. Although high separation efficiency has been achieved, the fundamental role of mechanical forces and electron transfer mechanisms in the mechanochemical process remains unclear. Therefore, further studies involving different types of cathode materials are essential to elucidate the underlying reaction mechanisms at the molecular level and expand the applicability of this approach.

Here, we propose a mechanochemical approach to convert LFP and LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) from spent LIBs by leveraging their intrinsic redox reactivity, driven by mechanical energy without the need for external chemical reagents. This mechanochemical process first induces the transformation of NCM and LFP to FePO₄, MnO, CoO, NiO, and Li₂O. The resulting ball-milled products then allow for the efficient separation of Li⁺, transition metals, and FePO₄ in aqueous and mildly acidic media. Process parameters were systematically optimized to assess feasibility, while the underlying mechanochemical transformation mechanisms—structural evolution, changes in the electronic structure, and thermodynamic driving forces—were elucidated through material characterization and density functional (DFT) calculations. Furthermore, life cycle assessment (LCA) was conducted to evaluate the environmental and economic sustainability of the proposed method. Overall, this work presents a sustainable recycling strategy that offers a mechanically controlled transformation pathway for the recovery of valuable resources from mixed spent LIBs.

2. Materials and methods

2.1. Materials and reagents

Spent LIBs were sourced from a local recycling center (Changsha, China). The batteries were manually disassembled to retrieve the cathode materials, which were then immersed in N-methylpyrrolidone (NMP) and subjected to ultrasonic cleaning for 60 min. The cleaned cathodes were subsequently dried in an oven at 105 °C for 1 h, yielding purified LFP and NCM powders. All chemical reagents, including hydrochloric acid (HCl, AR, 36%–38%), sulfuric acid (H₂SO₄, AR, 95%–98%), and nitric acid (HNO₃, AR, 65%–68%), were purchased from Sinopharm Chemical Reagent Co. Ltd.

2.2. Experimental procedure

NCM and LFP powders, with molar ratios from 0.6 : 1 to 1.4 : 1 (LFP : NCM), were initially homogenized using a zirconia mortar. The premixed powder was then transferred to a nylon milling tank without the addition of reagents. The effects of grinding time, rotation speed, ball-to-powder ratio, and LFP : NCM molar ratio on mechanochemical transformation were systematically investigated using a planetary ball mill (MSK-SFM-3, Hefei Kejing Materials Technology Co. Ltd). To mitigate heat accumulation, ball milling was interrupted for 10 min after every 30 min of operation, with the rotation direction reversed before resumption. The total milling time was determined based on the effective operating duration. Following milling, the samples were collected for subsequent leaching.

The ball-milled samples (MP-sample) were subjected to leaching under controlled conditions. A typical leaching procedure involved adding 1.0 g of the MP-sample into dilute H₂SO₄ or H₂O, followed by heating to the desired temperature for a predetermined duration. Upon completion of the reaction, the solution was immediately filtered, and the residues were dried in a vacuum oven at 105 °C for 5 h. The influence of key reaction parameters, including temperature, grinding time, and H₂SO₄ concentration, was systematically investigated. A schematic representation of the proposed mechanochemical process is provided in Fig. S1.† The metal leaching efficiency is determined using eqn (1):

where η (%) represents the metal leaching rate, C_i (g L⁻¹) and V (L) denote the lixivium concentration and volume, respectively, and m₀ (g) and ω_i (wt%) correspond to the weight of ball-milled products and the mass fraction of target metals, respectively.

2.3. Analytical methods

The elemental composition and metal concentrations were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer, USA). The structural and morphological characteristics of the samples were examined via X-ray diffraction (XRD, D8 Advance, Bruker, Germany), BET surface area analysis (Brunauer–Emmett–Teller, ASAP 2460, Micromeritics, USA), scanning electron microscopy (SEM, S4800, Hitachi, Japan), and transmission electron microscopy (TEM, Tecnai G2 Spirit, FEI, USA). The surface chemical states and composition changes were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA). To gain insight into the electronic structure and bonding characteristics, DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP 6.0) (Text S1†).

3. Results and discussion

3.1. Structural transformation during the mechanochemical processing

Scheme 1 depicts the proposed redox reaction pathway involving NCM and LFP during the MP; Fe(II) is oxidized to Fe(III), and Ni(III)/Ni(II), Co(III), and Mn(IV) in NCM are reduced to lower oxidation states (e.g., NiO, CoO and MnO). Various characterization techniques were employed to investigate the conversion behavior of metals during the MP. A significant increase in the specific surface area from 6.577 m² g⁻¹ to 10.532 m² g⁻¹ was observed after the MP, attributed to the structural collapse of the sample structure and the formation of new surfaces with more exposed reactive sites (Fig. 1a). Pore width distribution analysis revealed the presence of mesopores with larger pore sizes (2–5 nm), which facilitated mass transport in the liquid phase during the leaching process (Fig. S2†). The rotation speed played a crucial role in determining the particle size distribution. At a low rotation speed (e.g. 450 rpm), the powders exhibited a broad particle size distribution ranging from 200 to 1400 nm. In contrast, excessive rotation speed led to particle agglomeration, resulting in a wider distribution and an increase in average particle size (Fig. 1b and c). Therefore, an optimal rotation speed of 750 rpm was identified to produce fine and well-dispersed MP-samples with improved spreading and infiltration characteristics. The XRD patterns, as shown in Fig. 1d–f, indicate that the crystal structure of the sample was disrupted and progressively became amorphous with increasing rotation speed and milling duration. Additionally, the increased full width at half maximum (FWHM) at major Bragg planes – (101), (111), (311), (101), (104), and (003) – indicates crystallite size reduction, as determined via the Williamson–Hall method.³⁵ These findings confirm that the mechanical force altered the crystal properties of NCM and LFP during MP, leading to lattice distortion, grain refinement and amorphization, which in turn enhanced the reactivity of the mixture. SEM images of MP-samples revealed a loose and irregular structure with small particles, indicative of mechanically induced transformation (Fig. 1g and h). Furthermore, TEM analysis of the MP-samples (Fig. 1i–k) showed lattice disorder, with interlayer spacing of 0.280–0.285 nm, 0.276–0.278 nm and 0.369–0.374 nm, corresponding to Ni₂O₃ (002)/Co₃O₄ (220), LFP (301)/Mn₂O₃ (104)/Mn₃O₄ (103), and FePO₄ (120)/Li₃PO₄ (111), respectively. The available evidence suggests that the disruption of surface and lattice structures was accompanied by the formation of new phases, further confirming the mechanochemical transformation.

Scheme 1 Schematic diagram of the ball milling process.

Fig. 1 (a) N₂ adsorption–desorption curves of samples before and after MP; (b) particle size distribution and (c) average particle size variation of samples with different rotation speeds; XRD patterns of MP products under different conditions: (d) rotation speed, (e) milling time, and (f) the intensity and FWHM of samples under different rotation speeds; SEM images of products after MP: (g) 5 μm and (h) 500 nm; and TEM images of products after MP: (i) 500 nm, (j) 50 nm and (k) 5 nm. MP conditions: LFP : NCM at a molar ratio of 1 : 1, grinding time of 7 h and rotation speed of 750 rpm.

To elucidate the formation mechanism of MP samples, XPS analysis was conducted to investigate the valence state transitions of metals during MP (Fig. 2 and S3 and Tables S1–S4†). The Fe 2p spectrum exhibits asymmetric splitting into two components (Fe 2p_3/2 and Fe 2p_1/2) with a spin–orbit splitting energy of approximately 13.1 eV. The characteristic binding energies of Fe 2p at 710.3 eV (P1) and 723.6 eV (P3) were assigned to Fe(II) in LFP, while the peaks at 711.8 eV (P2) and 725.2 eV (P4) corresponded to Fe(III).³⁶ The presence of 22.69% Fe(III) in LFP was attributed to irreversible degradation during LIB cycling,³⁷ which is further supported by the appearance of FePO₄ signals in the XRD patterns of the subsequent leaching residues. The increased peak area of Fe(III) at higher rotation speeds suggests possible oxidation of Fe(II), indicating that the extent of Fe(III) oxidation increases under the influence of mechanical force. However, at an excessive rotation speed (e.g. 850 rpm), the Fe(III) peak intensity slightly decreased, possibly due to sample agglomeration at high mechanical energy, which reduced the effective contact between LFP and NCM and thereby limited the extent of redox interactions (Fig. 2a, Table S1†). This confirms the conversion from Fe(II) in LFP to Fe(III) after MP, which is consistent with the characteristic oxidation state of FePO₄. Similar to the Fe 2p spectrum, the Ni 2p and Co 2p spectra were split into two components: 2p_3/2 and 2p_1/2, with splitting energies of approximately 17.3 eV and 15–16 eV, respectively (Fig. 2b and c). The Ni 2p_3/2 and Ni 2p_1/2 spectra were deconvoluted into two primary peaks, confirming the coexistence of Ni(II) and Ni(III), with Ni(II) accounting for nearly 70% of the waste NCM.³⁸ As the rotation speed increased, the valence states of Ni and Co species shifted from +3 to +2, which contrasted with the oxidation of Fe(II) to Fe(III). Notably, the multiplet splitting in the Ni 2p_3/2 spectrum was most pronounced in NiO, approaching a single-split pattern, which suggested Ni(II) oxidation to higher-valence oxides during MP. Detailed information is shown in Tables S2 and S3.† The Mn 3s spectrum exhibited a doublet peak structure, attributed to parallel spin coupling between electrons in the 3s and 3d orbitals³⁹ (Fig. 2d). The splitting energy of Mn 3s (ΔE_3s) exhibited a linear trend with valence state changes (e.g. ΔE_3s-MnO = 6.1 eV, ΔE_3s-MnO(OH) = 5.4 eV, and ΔE_3s-MnO2 = 4.4 eV).⁴⁰ The calculated splitting energy (Table S4†) ranged from 4.4 to 6.1 eV, indicating the coexistence of Mn(II), Mn(III) and Mn(IV) in the MP samples. The average oxidation state (AOS) was calculated to further evaluate Mn valence changes (eqn (2) and (3)). The AOS significantly decreased from +3.59 to +2.18 as the rotation speed increased to 750 rpm, indicating electron transfer on the Mn surface during MP. Additionally, only a single P 2p doublet was observed, consistent with PO₄³⁻, with no significant change before and after MP, demonstrating the structural stability of phosphate.²⁸ These results conclusively demonstrate that the oxidation of Fe was coupled with the reduction of Ni, Co, and Mn, while P remained stable in the form of phosphate. A possible chemical reaction equation is given as eqn (4). However, we fully acknowledge that this reaction represents a simplified reaction hypothesis based on thermodynamic considerations and indirect experimental evidence, rather than a fully confirmed stoichiometric conversion.

AOS_Mn = 9.67 − 1.27ΔE_3s (2)

ΔE_3s = site (P1) − site (P2) (3)

Fig. 2 XPS spectra of Fe 2p (a), Ni 2p (b), Co 2p (c), and Mn 2p (d) in products with different milling rotation speeds (0 rpm, 450 rpm, 550 rpm, 650 rpm, 750 rpm and 850 rpm).

Building upon analysis, layered NCM adopts a hexagonal α-NaFeO₂-type structure with an R3̄m space group, consisting of alternating layers of metal ions (Me = Ni, Co, and Mn) and Li ions, separated by oxygen atomic layers.^41,42 Within this structure, octahedral MeO₆ units are arranged in a two-dimensional Me–O layer by sharing octahedral side edges, while Li⁺ occupies octahedral LiO₆ sites between Co–O layers through ionic interactions with adjacent oxygen atoms. Unlike NCM, LFP exhibits an olivine-type symmetric octahedral structure with a compact hexagonal oxygen arrangement. In this framework, the Fe center is coordinated by six oxygen atoms to form Fe-centered octahedral FeO₆ units, while the P center is tetrahedrally coordinated by four oxygen atoms to form PO₄ units.

These structural components form a robust framework with a chain-like structure. During ball milling, the materials inside the milling jar experience a combination of shear forces, pressure, and impact forces, leading to the extraction of Li⁺ from the LiO₆ units in the LFP olivine framework and the collapse of the layered NCM structure. This process promotes the formation of Li₂O and amorphous phases, while reducing the particle size. Mechanical energy is transmitted from the zirconia grinding balls to the contents of the milling jar, enhancing the reactivity of solid materials. The regeneration of fresh surfaces, high-energy collision sites, and intense internal energy input induces lattice vibration, lattice distortions, and atomic rearrangements. These factors facilitate the formation of transition metal oxides (MeO) and the in situ conversion of Fe(II) to Fe(III) within FePO₄, facilitated by Li extraction. Not all reactions strictly adhere to the proposed idealized pathways, and some intermediates, such as Co₃O₄, Ni₂O₃, and Mn₃O₄, may form as a result of additional side reactions (see eqn (5)–(14)). Based on the standard thermodynamic data available in Table S5,† the Gibbs free energy values (Δ_rG_m) for each potential reaction are detailed in Table S6.†

2LiFePO₄ + Co₃O₄ → 2FePO₄ + 3CoO + Li₂O (6)

2LiFePO₄ + Mn₃O₄ → 2FePO₄ + 3MnO + Li₂O (7)

2LiFePO₄ + Ni₂O₃ → 2FePO₄ + 2NiO + Li₂O (8)

3LiFePO₄ + Co₃O₄ → Li₃PO₄ + 3CoO + 2FePO₄ + FeO (11)

3LiFePO₄ + Mn₃O₄ → Li₃PO₄ + 3MnO + 2FePO₄ + FeO (12)

3LiFePO₄ + Ni₂O₃ → Li₃PO₄ + 2NiO + 2FePO₄ + FeO (13)

3.2. Leaching behaviours of different metals after mechanochemical processing

3.2.1. Parameter optimization. Theoretically, the mechanochemically induced redox reaction between NCM and LFP facilitates the transformation and redistribution of elements (Fig. S4†). To analyze the composition of MP samples, water was first used as a leaching agent to separate metals (Fig. S5†). Increasing the rotation speed or grinding time enhanced the Li leaching efficiency (∼80%), while no transition metals were detected, likely due to the incomplete conversion of NCM and LFP. To further disrupt the structure of MP samples and enhance metal extraction, a mildly acidic system was introduced. Interestingly, the leaching behavior changed significantly after MP, with Li, Ni, Co, and Mn detected in the filtrate even when the acid dosage was below the stoichiometric requirement of H₂SO₄ (Fig. S6†). The unexpected solubility of Ni, Co and Mn could be attributed to the reduction of NCM to acid-soluble Ni²⁺, Co²⁺ and Mn²⁺ and the concurrent formation of insoluble FePO₄. MP significantly improved the leaching process by selectively extracting Li, Ni, Co and Mn while stabilizing Fe and P as insoluble residues (Fig. 3 and S7†). Excessive acid concentration reduces the leaching efficiency of Fe and P (Fig. S7†), likely due to enhanced metal dissolution and redox reactions. Under slightly acidic conditions, Fe²⁺ oxidizes to Fe³⁺, forming insoluble FePO₄ precipitates, while Ni, Co, and Mn undergo reduction and leaching.¹⁸ The overall reaction occurring in solution can be described as eqn (15).

Fig. 3 Leaching behaviours of Li, Ni, Co, Mn, Fe, and P with and without mechanical processing.

Highly selective leaching of valuable metals was achieved using only a stoichiometric acid concentration (0.16 mol L⁻¹), resulting in increases of approximately 30% and 60% in the leaching efficiencies of Li and Ni, Co and Mn, respectively, compared to samples without MP. Insufficient leaching time or temperature (e.g., 70 °C, 1 h) led to incomplete reactions between NCM and LFP, whereas excessively high values did not further enhance leaching efficiency but instead led to unnecessary energy consumption (Fig. S8†). Therefore, the optimal leaching conditions were determined to be 1.5 h at 90 °C. The effect of mechanical forces on the conversion of NCM and LFP was further evaluated by controlling key grinding variables, including rotation speed, grinding time, ball-to-powder ratio and LFP : NCM molar ratio (Fig. S8 and S9†). Increasing the rotation speed or grinding time enhanced the leaching efficiency of Li, Ni, Co and Mn due to the greater mechanical energy input. However, excessive rotation speed or grinding time slightly decreased the leaching efficiency due to sample agglomeration, which hindered mass transport in the liquid phase.^9,43 The ball-to-powder ratio positively influenced the leaching efficiency of all elements by ensuring sufficient particle collisions (Fig. S8†). Additionally, the LFP : NCM molar ratio played a crucial role in leaching efficiency. When the ratio was between 0.6 : 1 and 1 : 1, LFP was fully converted, facilitating the precipitation of Fe³⁺. However, an excessive LFP dosage led to the incomplete oxidation of Fe²⁺ (Fig. S8†). To optimize leaching efficiency while minimizing energy consumption, the optimal recovery conditions were determined as follows: a rotation speed of 750 rpm, a grinding time of 7 h, a ball-to-powder ratio of 50 : 1, and an LFP : NCM molar ratio of 1 : 1. Under these conditions, the maximum leaching efficiencies of Li, Ni, Co, and Mn reached 99.99%, 79.69%, 86.68%, and 88.57%, respectively, with materials including NCM811, NCM333, and LCO (Fig. S10†).

The results demonstrated that valuable metals could be selectively extracted with good universality, while Fe and P were simultaneously precipitated. The intrinsic reducibility of LFP effectively promotes the reduction of high-valence metals, confirming the feasibility of achieving selective leaching of valuable metals using stoichiometric acid. Additionally, the transformation of P and Fe into insoluble residues enabled a one-step separation and recovery of Li, Ni, Co, and Mn from lixivium, while Fe and P remained in the solid residues.

3.2.2. Identification of the separated residue. To improve phase stability and crystallinity, the residues were calcined at 500 °C. A theoretical model based on crystalline transition metal oxides was employed to assess the thermodynamic feasibility of the reaction, serving as a reasonable approximation (Fig. S11†), despite the partial amorphization observed in the mechanochemical products. For Li⁺, the recycled Li₂CO₃ exhibited XRD patterns consistent with the standard diffraction pattern of Li₂CO₃ (PDF#00-022-1141) (Fig. S12†). Li occupies a minor proportion in the partial density of states (PDOS) (Fig. 4a). The Li–O bond length in Li₂O is shorter, indicating a more stable configuration that facilitates electron loss from LFP and NCM111 (Fig. 4b and Table S7†). The XRD patterns of water leaching residues were consistent with the standard diffraction card of FePO₄ powders, with the presence of transition metal oxides (e.g., NiO, CoO and MnO) (Fig. S13†). SEM and EDS analyses revealed that the residues consisted of loosely packed, irregular particles with the atom composition determined as C (37.93 at%), O (46.60 at%), Fe (7.31 at%) and P (6.68 at%) (Fig. S14 and S15†). Compared with the reaction products, NCM111 exhibits a higher density of electronic states near the Fermi level, along with greater electronic delocalization, making it more susceptible to activation (Fig. 4c). NCM and its corresponding transition metal oxides (NiO, CoO and MnO) exhibit strong hybridization between Ni, Co, and Mn orbitals with O, along with extensive orbital overlap, confirming the presence of robust covalent bonding in Me–O (Me = Ni, Co, Mn) (Fig. 4d and e). The increased band gap after the reaction suggests enhanced structural stability. Additionally, both the total DOS near the Fermi level and the peak splitting in NCM are greater than those of the resulting metal oxides, further supporting the stability of the reaction products. For FePO₄, the XPS results revealed that the Fe 2p XPS spectrum exhibits characteristic peaks of Fe(III) at 725.28 eV and 712.08 eV. The P 2p spectrum suggested that the PO₄³⁻ structure remained stable and the coordination environment of P was unchanged, with no new phosphide formed in the leaching residues (Fig. S16†). The atom ratio of Fe to P was 1 : 1, which was consistent with the XRD analysis. According to DFT calculations, the Fe–O bond length in FePO₄ is shorter, indicating a more stable configuration that facilitates electron loss from LFP (Fig. 4f). The primary contribution near the Fermi level originates from Fe orbitals in LFP, which promotes oxidation by enhancing electron mobility. In contrast, the PDOS of FePO₄ suggests that O orbitals dominate near the Fermi level, indicating greater electronic activity than Fe orbitals (Fig. 4g and h). This implies that Fe in FePO₄ is more stable than Fe in LFP. Structural optimization calculations revealed that the PO₄ coordination environment remains consistent between LFP and FePO₄, preserving the structural integrity of the phosphate framework (Fig. S17†). Moreover, the weak interaction between Li and surrounding atoms results in a negligible contribution of Li to the DOS of LFP. In both LFP and FePO₄, the substantial Fe–O orbital overlap indicates strong hybridization and covalent bonding, allowing Fe(II) to be oxidized in situ to Fe(III) while remaining within the tetrahedral PO₄ framework, thereby preserving structural stability. These findings collectively indicate that the stoichiometric acid leaching process resulted in the selective extraction of Li, Ni, Co, and Mn, while Fe and P remained as insoluble FePO₄ precipitates. Subsequently, a well-crystallized Ni_0.5Co_0.2Mn_0.3(OH)₂ precursor featuring a uniform nanoflower-like structure was synthesized via co-precipitation (Fig. S18†). Overall, the conversion of LFP and NCM into FePO₄ and NiO/CoO/MnO is thermodynamically favorable, driven by the increased structural stability of the products, optimized bond lengths, and an electronic configuration that promotes redox reactions.

Fig. 4 Recovery of Li⁺: (a) partial density of states of Li₂O and (b) configurations of Li in LFP, NCM111 and Li₂O. Recovery of Ni²⁺, Co²⁺ and Mn²⁺: total density of states of (c) NCM111 and partial density of states of (d) NCM111 and (e) MnO, CoO, and NiO. Conversion of FePO₄: (f) configurations of Fe in LFP and FePO₄, (g) total density of states of LFP and FePO₄, and (h) partial density of states of LFP and FePO₄.

3.3. Techno-economic analysis of different recycling technologies

Fig. 5a summarizes the main recycling strategies for spent LIBs containing LFP and NCM cathodes, including the pyro-, hydro-, and mechanochemical methods proposed in this study. Detailed flowcharts of the three processes are presented in Fig. S19–S21.† A comparative analysis of the cost, revenue, and energy consumption for recycling 1 kg of spent LFP and NCM batteries (yielding only 0.27 kg of NCM and 0.18 kg of LFP active materials) is provided in Table S8.† According to the EverBatt 2023 model, the recycling costs of the pyro-, hydro-, and mechanochemical (this work) methods were estimated to be 5.77, 4.82 and 2.13 $ per kg feedstock, respectively (Fig. 5b and Table S9†). It is worth noting that the annualized capital cost is a critical factor across all technologies, while high fixed and maintenance costs make the pyro-process particularly expensive. For the product value, only the mechanochemical method enables the selective separation and regeneration of LFP and NCM cathode materials, whereas the pyro- and hydro-routes typically yield low-value alloys or semi-finished products, substantially diminishing their economic return (Fig. 5c). Notably, regenerated NCM is the primary revenue contributor in the ball milling route, delivering a profit of 8.87 $ per kg feedstock. From an environmental perspective, the mechanochemical route shows significant advantages in green chemistry dimensions. This process minimizes chemical usage by avoiding strong oxidants, reductants, or organic extractants, while directly immobilizing Fe and P as solid FePO₄ residue. As shown in Fig. 5d and Table S10,† the total emissions for the pyro-, hydro-, and mechanochemical processes were 0.36, 4.75 and 0.10 kg per kg feedstock processed, respectively, highlighting the environmental advantage of the ball milling route. In summary, we conducted a techno-economic assessment of pyro-, hydro-, and mechanochemical recycling under ideal circumstances. The comparison indicated that the ball milling approach outperforms conventional methods in terms of energy savings, cost reduction, operational simplicity, and economic profitability, making it a promising and sustainable alternative for the recycling of spent LIBs (Fig. 5e and Tables S11 and S12†). Although mechanical activation contributes to the overall energy input, its role in inducing redox reactions and selective leaching is crucial. To further improve energy efficiency, process parameters (e.g., milling duration, rotation speed and ball-to-powder ratio) can be optimized to minimize unnecessary energy consumption while maintaining activation efficiency. In addition, advanced milling equipment may offer further reductions in energy demand. These optimization efforts provide a clear pathway for future technology iteration and industrial scalability.

Fig. 5 (a) Schematic of the pyro-, hydro-, and ball-milling technologies. (b) The cost, (c) revenue, (d) total emissions, and (e) comparison of the different recycling processes.

4. Conclusions

A mechanochemical process with minimal chemical consumption was developed for the efficient and selective recovery of metals from mixed cathode materials. Through the combined insights of theoretical modelling and experimental validation, we demonstrated that mechanical forces drive the selective transformation of NCM and LFP, enabling controlled reaction pathways and recoverable product formation. Under mechanical activation, lattice distortion and atomic rearrangement facilitate the in situ conversion of LFP to FePO₄, while reducing NCM to low-valence transition metal oxides (e.g., NiO, MnO, and CoO). This process leverages the intrinsic redox properties of cathode materials, allowing controlled reactant input, mechanochemically guided phase transformation, and the conversion of metals into easily recoverable states. By minimizing reagent use and simplifying separation steps, this approach offers a sustainable and economically viable pathway for LIB cathode recycling, contributing to the development of greener and more efficient battery recycling technologies.

Author contributions

Shubin Wang: investigation, data curation, and writing – original draft; Shuxuan Yan: methodology, data curation, and writing – original draft; Zihao Chen: data curation; Yudie Ou: formal analysis and data curation; Binod Mahara: data curation; Xiangping Chen: methodology, conceptualization, supervision, funding acquisition, and writing – review & editing; Ying Yang: supervision; and Tao Zhou: methodology, supervision, funding acquisition, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

All the data supporting this article have been included in the main text and ESI.†

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52074177 and 52174391), the Science and Technology Innovation Program of Hunan Province (No. 2023RC3129), and the Natural Science Foundation of Hunan Province (No. 2023JJ20031). This work was supported in part by the High Performance Computing Center of Central South University. All the authors also appreciate the editor(s) and anonymous reviewer(s) with gratitude for their professional comments and constructive suggestions.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5gc02354h

‡ These authors contributed equally to this work.

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